Thanks toptrish and A1.
I have a (ballofix type) isolation valve - such an obvious way to reduce the flow-rate if necessary but I never thought of it! - but should I have to do that?
I have a multimeter so, presumably, I need to switch on the shower at hottest and measure the current - about 40A you say - will have to check how to do that - tend to use it mainly for continuity or voltage. Also what is a stepper motor, does this shower have one and, if so, what do I do?

Tony Lee, January 2014

You can't test that amount of power with a multimeter,is your multimeter an analogue type or digital, if digital the resistance can be checked,if analogue a circuit will have to do.
on top of the heater tank are 4 connections 2 are bonded together just test the left 2 with meter on ohms setting to see if there's a circuit, at the bottom of tank there is a cut out with 2 blue wires,test for continuity.
the stepper motor is on the right it controls the amount of water flowing through the heat exchanger, the only way you can test it is by turning the temperature dial up and down to see if the water flow changes.
log onto www.showerspares .com then spares and locate your shower this will show a blown up view of your shower

toptrish, January 2014